Revert "Issue #1189804 by cosmicdreams, jessebeach, mgifford, pwieck, smiro, dcmouyard: Convert aggregator-feed-source.html.twig to HTML5."

This reverts commit 5ca04d6782ea04b04f3b66b5ee19013e080c0905.
Issue #2143519 by yched, Wim Leers: Allow FieldInstance yml files to refer to the Field by field name rather than by field_uuid.